1. Field of the Invention
The present invention relates to an apparatus and method for demodulating a modulated signal.
2. Description of Related Art
Bandpass sampling techniques are popular in the field of digital intermediate frequency (D-IF) receivers. In such techniques, a main consideration is to reduce the power consumption by using a lower sampling rate. With bandpass sampling, the sampling rate can be lower than the IF frequency as long as it is higher than two times the bandwidth of a signal being processed.
Conventional bandpass sampling techniques typically oversample the signal being processed whilst undersampling the IF signal. The sampling rate of bandpass sampling should meet the following requirement:
                                          2            ⁢                          f              U                                n                ≤                  f          S                ≤                                            2              ⁢                              f                L                                                    n              -              1                                ⁢                                          ⁢          1                ≥        n        ≤                  ⌊                                    f              U                        B                    ⌋                                    (        1        )            Where fU is the highest frequency of the IF signal, fL is the lowest frequency of the IF signal, fS is the sampling rate, and B is the occupied bandwidth of the IF signal. The equivalent digital IF after sampling will be denoted by:fDIF=(fIF−n*fs)  (2)where fIF is the analog IF of the received signal and n is an integer. Normally, the sampling rate will be set as an integer multiple of the symbol rate. Also, there are some practical considerations in the selection of analog IF signals. For example, if a conventional D-IF receiver has the following parameters fIF=40 MHz, B=5 MHz, fsym (the data symbol rate)=2.048M; the lowest sampling rate equal to the integer multiple of the symbol rate will be 12.288 MHz (n=6) according to equation (1). The equivalent IF signal after sampling is denoted by equation (2) as follows:fDIF=(fIFn*fs)=(40−3*12.288)=3.136 MHz
In traditional demodulators for M-ary Phase Shift Keying (MPSK) and Differential M-ary Phase shift Keying (DMPSK) signals, an analog to digital converter (ADC) is used to convert a received analog signal (either a baseband signal or an intermediate frequency (IF) signal) into digital form for further processing. For digital processing, the signal is passed through a digital down converter (DDC) and a low pass filter (LPF) before being demodulated. A problem with the hardware typically used in the conventional bandpass sampling techniques is that the DDC and the following LPF structure cannot generally be simplified.
Therefore, there is a need for a less complex and more economical apparatus and method for bandpass sampling.